Handoff procedure in a mobile communication system

ABSTRACT

The invention relates to an improved handover procedure for a mobile terminal. Under control of the target base station, the mobile terminal is to perform a handoff to a target base station, wherein it is to be configured for communication with the target base station via a target radio cell comprising a downlink carrier and an uplink carrier. The mobile terminal receives a handoff command message for the handoff to the target base station including a handoff execution condition as trigger for executing handoff to the target base station. Then, the mobile terminal determines, based on the received handoff execution condition, whether or not the mobile terminal is to trigger execution of the handoff to the target base station. In case the mobile terminal determines that it is to trigger execution of the handoff to the target base station, the mobile terminal executes the handoff to the target base station.

FIELD OF THE INVENTION

The invention relates to methods for performing a handoff of a mobileterminal to a target base station. As part of the handoff, the mobileterminal is to be configured for communication with the target basestation via a target radio cell comprising a downlink and an uplinkcarrier. The invention also provides the mobile terminal and the basestation for performing the methods described herein.

TECHNICAL BACKGROUND

Long Term Evolution (LTE)

Third-generation mobile systems (3G) based on WCDMA radio-accesstechnology are being deployed on a broad scale all around the world. Afirst step in enhancing or evolving this technology entails introducingHigh-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, alsoreferred to as High Speed Uplink Packet Access (HSUPA), giving a radioaccess technology that is highly competitive.

In order to be prepared for further increasing user demands and to becompetitive against new radio access technologies, 3GPP introduced a newmobile communication system which is called Long Term Evolution (LTE).LTE is designed to meet the carrier needs for high speed data and mediatransport as well as high capacity voice support for the next decade.The ability to provide high bit rates is a key measure for LTE.

The work item (WI) specification on Long-Term Evolution (LTE) calledEvolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial RadioAccess Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTEsystem represents efficient packet-based radio access and radio accessnetworks that provide full IP-based functionalities with low latency andlow cost. In LTE, scalable multiple transmission bandwidths arespecified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order toachieve flexible system deployment using a given spectrum. In thedownlink, Orthogonal Frequency Division Multiplexing (OFDM) based radioaccess was adopted because of its inherent immunity to multipathinterference (MPI) due to a low symbol rate, the use of a cyclic prefix(CP) and its affinity to different transmission bandwidth arrangements.Single-carrier frequency division multiple access (SC-FDMA) based radioaccess was adopted in the uplink, since provisioning of wide areacoverage was prioritized over improvement in the peak data rateconsidering the restricted transmit power of the user equipment (UE).Many key packet radio access techniques are employed includingmultiple-input multiple-output (MIMO) channel transmission techniquesand a highly efficient control signaling structure is achieved in LTERel. 8/9.

LTE Architecture

The overall architecture is shown in FIG. 1 and a more detailedrepresentation of the E-UTRAN architecture is given in FIG. 2. TheE-UTRAN consists of an eNodeB, providing the E-UTRA user plane(PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towardsthe user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY),Medium Access Control (MAC), Radio Link Control (RLC) and Packet DataControl Protocol (PDCP) layers that include the functionality ofuser-plane header-compression and encryption. It also offers RadioResource Control (RRC) functionality corresponding to the control plane.It performs many functions including radio resource management,admission control, scheduling, enforcement of negotiated uplink Qualityof Service (QoS), cell information broadcast, ciphering/deciphering ofuser and control plane data, and compression/decompression ofdownlink/uplink user plane packet headers. The eNodeBs areinterconnected with each other by means of the X2 interface.

The eNodeBs are also connected by means of the S1 interface to the EPC(Evolved Packet Core), more specifically to the MME (Mobility ManagementEntity) by means of the S1-MME and to the Serving Gateway (SGVV) bymeans of the S1-U. The S1 interface supports a many-to-many relationbetween MMEs/Serving Gateways and eNodeBs. The SGW routes and forwardsuser data packets, while also acting as the mobility anchor for the userplane during inter-eNodeB handovers and as the anchor for mobilitybetween LTE and other 3GPP technologies (terminating S4 interface andrelaying the traffic between 2G/3G systems and PDN GVV). For idle stateuser equipments, the SGW terminates the downlink data path and triggerspaging when downlink data arrives for the user equipment. It manages andstores user equipment contexts, e.g. parameters of the IP bearerservice, network internal routing information. It also performsreplication of the user traffic in case of lawful interception.

The MME is the key control-node for the LTE access-network. It isresponsible for idle mode user equipment tracking and paging procedureincluding retransmissions. It is involved in the beareractivation/deactivation process and is also responsible for choosing theSGW for a user equipment at the initial attach and at time of intra-LTEhandover involving Core Network (CN) node relocation. It is responsiblefor authenticating the user (by interacting with the HSS). TheNon-Access Stratum (NAS) signaling terminates at the MME and it is alsoresponsible for generation and allocation of temporary identities touser equipments. It checks the authorization of the user equipment tocamp on the service provider's Public Land Mobile Network (PLMN) andenforces user equipment roaming restrictions. The MME is the terminationpoint in the network for ciphering/integrity protection for NASsignaling and handles the security key management. Lawful interceptionof signaling is also supported by the MME. The MME also provides thecontrol plane function for mobility between LTE and 2G/3G accessnetworks with the S3 interface terminating at the MME from the SGSN. TheMME also terminates the S6a interface towards the home HSS for roaminguser equipments.

Handover Procedure

The term Connected Mode Mobility refers to various procedures e.g.handover procedure. In particular, a 3GPP LTE handover procedure isspecified in 3GPP TS 36.300: “Evolved Universal Terrestrial Radio Access(E-UTRA) and Evolved Universal Terrestrial Radio Access Network(E-UTRAN); Overall description; Stage 2”, version 11.5.0, section 10.1.2available at http//www.3gpp.org and incorporated herein by reference.Further, details of the handover procedure relating to RRC connectionreconfiguration are defined in TS 36.331: “Evolved Universal TerrestrialRadio Access (E-UTRA); Radio Resource Control (RRC); Protocolspecification)”, version 11.4.0 section 5.3.5 available athttp//www.3gpp.org and incorporated herein by reference.

The Intra-E-UTRAN-Access Mobility Support for UEs in CONNECTED Modehandles all necessary steps for handover procedures, like processes thatprecede the final handover, HO, decision on the source network side(control and evaluation of UE and eNB measurements taking into accountcertain UE specific area restrictions), preparation of resources on thetarget network side, commanding the UE to the new radio resources andfinally releasing resources on the (old) source network side.

The intra E-UTRAN handover, HO, of a UE in RRC_CONNECTED state is aUE-assisted network-controlled HO, with HO preparation signalling inE-UTRAN:

-   -   Part of the HO command comes from the target eNB and is        transparently forwarded to the UE by the source eNB;    -   To prepare the HO, the source eNB passes all necessary        information to the target eNB.    -   Both the source eNB and UE keep some context (e.g. C-RNTI) to        enable the return of the UE in case of HO failure;    -   UE accesses the target cell via RACH following a contention-free        procedure using a dedicated RACH preamble or following a        contention-based procedure if dedicated RACH preambles are not        available:    -   the UE uses the dedicated preamble until the handover procedure        is finished (successfully or unsuccessfully);    -   If the RACH procedure towards the target cell is not successful        within a certain time, the UE initiates radio link failure        recovery using the best cell;

The preparation and execution phase of the HO procedure is performedwithout EPC involvement, i.e. preparation messages are directlyexchanged between the eNBs. The release of the resources at the sourceside during the HO completion phase is triggered by the eNB. The figurebelow depicts the basic handover scenario; only the first few steps areexplained that are more relevant from this invention perspective:

Below a more detailed description of the intra-MME/Serving Gatewayhandover, HO, procedure illustrated in FIG. 3 is given where preceedingnumbers refer to corresponding steps in the sequence diagram of thefigure:

-   -   0 The UE context within the source eNB contains information        regarding roaming restrictions which were provided either at        connection establishment or at the last TA update.    -   1 The source eNB configures the UE measurement procedures        according to the area restriction information. Measurements        provided by the source eNB may assist the function controlling        the UE's connection mobility.    -   2 A MEASUREMENT REPORT is triggered and sent to the eNB.    -   3 The source eNB makes decision based on MEASUREMENT REPORT and        RRM information to hand off the UE.    -   4 The source eNB issues a HANDOVER REQUEST message to the target        eNB passing necessary information to prepare the HO at the        target side (UE X2 signalling context reference at source eNB,        UE S1 EPC signalling context reference, target cell ID, KeNB*,        RRC context including the C-RNTI of the UE in the source eNB,        AS-configuration, E-RAB context and physical layer ID of the        source cell+short MAC-I for possible RLF recovery). UE X2/UE S1        signalling references enable the target eNB to address the        source eNB and the EPC. The E-RAB context includes necessary RNL        and TNL addressing information, and QoS profiles of the E-RABs.    -   5 Admission Control may be performed by the target eNB dependent        on the received E-RAB QoS information to increase the likelihood        of a successful HO, if the resources can be granted by target        eNB. The target eNB configures the required resources according        to the received E-RAB QoS information and reserves a C-RNTI and        optionally a RACH preamble. The AS-configuration to be used in        the target cell can either be specified independently (i.e. an        “establishment”) or as a delta compared to the AS-configuration        used in the source cell (i.e. a “reconfiguration”).    -   6 The target eNB prepares HO with L1/L2 and sends the HANDOVER        REQUEST ACKNOWLEDGE to the source eNB. The HANDOVER REQUEST        ACKNOWLEDGE message includes a transparent container to be sent        to the UE as an RRC message to perform the handover. The        container includes a new C-RNTI, target eNB security algorithm        identifiers for the selected security algorithms, may include a        dedicated RACH preamble, and possibly some other parameters i.e.        access parameters, SIBs, etc. The HANDOVER REQUEST ACKNOWLEDGE        message may also include RNL/TNL information for the forwarding        tunnels, if necessary.

NOTE: As soon as the source eNB receives the HANDOVER REQUESTACKNOWLEDGE, or as soon as the transmission of the handover command isinitiated in the downlink, data forwarding may be initiated.

Steps 7 to 16 provide means to avoid data loss during HO and are furtherdetailed in 10.1.2.1.2 and 10.1.2.3.

-   -   7 The target eNB generates the RRC message to perform the        handover, i.e RRCConnectionReconfiguration message including the        mobilityControlInformation, to be sent by the source eNB towards        the UE. The source eNB performs the necessary integrity        protection and ciphering of the message. The UE receives the        RRCConnectionReconfiguration message with necessary parameters        (i.e. new C-RNTI, target eNB security algorithm identifiers, and        optionally dedicated RACH preamble, target eNB SIBs, etc.) and        is commanded by the source eNB to perform the HO. The UE does        not need to delay the handover execution for delivering the        HARQ/ARQ responses to source eNB.    -   8 The source eNB sends the SN STATUS TRANSFER message to the        target eNB to convey the uplink PDCP SN receiver status and the        downlink PDCP SN transmitter status of E-RABs for which PDCP        status preservation applies (i.e. for RLC AM). The uplink PDCP        SN receiver status includes at least the PDCP SN of the first        missing UL SDU and may include a bit map of the receive status        of the out of sequence UL SDUs that the UE needs to retransmit        in the target cell, if there are any such SDUs. The downlink        PDCP SN transmitter status indicates the next PDCP SN that the        target eNB shall assign to new SDUs, not having a PDCP SN yet.        The source eNB may omit sending this message if none of the        E-RABs of the UE shall be treated with PDCP status preservation.    -   9 After receiving the RRCConnectionReconfiguration message        including the mobilityControlInformation, UE performs        synchronization to target eNB and accesses the target cell via        RACH, following a contention-free procedure if a dedicated RACH        preamble was indicated in the mobilityControlInformation, or        following a contention-based procedure if no dedicated preamble        was indicated. UE derives target eNB specific keys and        configures the selected security algorithms to be used in the        target cell.    -   10 The target eNB responds with UL allocation and timing        advance.    -   11 When the UE has successfully accessed the target cell, the UE        sends the RRCConnectionReconfigurationComplete message (C-RNTI)        to confirm the handover, along with an uplink Buffer Status        Report, whenever possible, to the target eNB to indicate that        the handover procedure is completed for the UE. The target eNB        verifies the C-RNTI sent in the        RRCConnectionReconfigurationComplete message. The target eNB can        now begin sending data to the UE.    -   12 The target eNB sends a PATH SWITCH REQUEST message to MME to        inform that the UE has changed cell.    -   13 The MME sends a MODIFY BEARER REQUEST message to the Serving        Gateway.    -   14 The Serving Gateway switches the downlink data path to the        target side. The Serving gateway sends one or more “end marker”        packets on the old path to the source eNB and then can release        any U-plane/TNL resources towards the source eNB.    -   15 The Serving Gateway sends a MODIFY BEARER RESPONSE message to        MME.    -   16 The MME confirms the PATH SWITCH REQUEST message with the        PATH SWITCH REQUEST ACKNOWLEDGE message.    -   17 By sending the UE CONTEXT RELEASE message, the target eNB        informs success of HO to source eNB and triggers the release of        resources by the source eNB. The target eNB sends this message        after the PATH SWITCH REQUEST ACKNOWLEDGE message is received        from the MME.    -   18 Upon reception of the UE CONTEXT RELEASE message, the source        eNB can release radio and C-plane related resources associated        to the UE context. Any ongoing data forwarding may continue.

When an X2 handover is used involving HeNBs and when the source HeNB isconnected to a HeNB GW, a UE CONTEXT RELEASE REQUEST message includingan explicit GW Context Release Indication is sent by the source HeNB, inorder to indicate that the HeNB GW may release of all the resourcesrelated to the UE context.

Radio Link Failure

Radio Link Failures, RLF, have in the past been widely studied andcharacterized. Specifically, in the context of 3GPPP LTE, two differentphases govern the behavior associated to radio link failure.

A First Phase can be characterized as follows it is starts upondetection of a radio problem; it leads to radio link failure; itcorresponds to no UE-based mobility; it is distinguishable based ontimer or other (e.g. counting) criteria (T₁). A Second Phase can becharacterized as follows: it is started upon radio link failuredetection or handover failure; it leads to the UE switching to theRRC_IDLE state; it corresponds to UE-based mobility; and it isdistinguishable based on timer (T₂).

In the table below, it is described how mobility is handled with respectto radio link failure.

Cases First Phase Second Phase T2 expired UE returns to the sameContinue as if no Activity is resumed by Go via RRC_IDLE cell radioproblems means of explicit signaling occurred between UE and eNB UEselects a different N/A Activity is resumed by Go via RRC_IDLE cell fromthe same eNB means of explicit signaling between UE and eNB UE selects acell of a N/A Activity is resumed by Go via RRC_IDLE prepared eNB (NOTE)means of explicit signaling between UE and eNB UE selects a cell of aN/A Go via RRC_IDLE Go via RRC_IDLE different eNB that is not prepared(NOTE) (NOTE): a prepared eNB is an eNB which has admitted the UE duringan earlier executed HO preparation phase.

In the Second Phase, in order to resume activity and avoid going viaRRC_IDLE when the UE returns to the same cell or when the UE selects adifferent cell from the same eNB, or when the UE selects a cell from adifferent eNB, the following procedure applies:

-   -   The UE stays in RRC_CONNECTED;    -   The UE accesses the cell through the random access procedure;    -   The UE identifier used in the random access procedure for        contention resolution (i.e. C-RNTI of the UE in the cell where        the RLF occurred+physical layer identity of that cell+short        MAC-I based on the keys of that cell) is used by the selected        eNB to authenticate the UE and check whether it has a context        stored for that UE:    -   If the eNB finds a context that matches the identity of the UE,        it indicates to the UE that its connection can be resumed;    -   If the context is not found, RRC connection is released and UE        initiates procedure to establish new RRC connection. In this        case UE is required to go via RRC_IDLE.

The radio link failure procedure applies also for RNs, with theexception that the RN is limited to select a cell from its DeNB celllist. Upon detecting radio link failure, the RN discards any current RNsubframe configuration (for communication with its DeNB), enabling theRN to perform normal contention-based RACH as part of there-establishment. Upon successful re-establishment, an RN subframeconfiguration can be configured again using the RN reconfigurationprocedure.

If the recovery attempt in the second phase fails, the details of the RNbehavior in RRC_IDLE to recover an RRC connection are up to the RNimplementation.

Radio Link Failure is described in detail in 3GPP TS 36.300: “EvolvedUniversal Terrestrial Radio Access (E-UTRA) and Evolved UniversalTerrestrial Radio Access Network (E-UTRAN); Overall description; Stage2”, version 11.5.0, section 10.1.6 available at http//www.3gpp.org andincorporated herein by reference.

Technology Areas

This invention can be used as a solution for improving mobilityrobustness in many technologies including:

Heterogeneous Networks

A Heterogeneous network is generally referred in this invention fordeployment scenarios where the neighboring cells differ (sometimessubstantially) in transmit power/cell range/size. The bigger cell/eNB(in transmit power/cell range/size) is generally referred to as Macro(or Master, Main, Aggressor etc.) and the small cell/eNB is referred toas Small (or Pico, Secondary, Victim etc.). This invention further doesnot distinguishes within e.g. Small Cells/eNBs i.e. Pico and Small eNBare treated/named in a similar way irrespective of their definition (anddifference in real transmit power/size) elsewhere—this invention appliesfor all scenarios with mixed cell/eNB deployments.

Small Cells

Explosive demands for mobile data are driving changes in how mobileoperators will need to respond to the challenging requirements of highercapacity and improved Quality of user Experience (QoE). Currently,fourth generation wireless access systems using Long Term Evolution(LTE) are being deployed by many operators worldwide in order to offerfaster access with lower latency and more efficiency than 3G/3.5Gsystem. Nevertheless, the anticipated future traffic growth is sotremendous that there is a vastly increased need for further networkdensification to handle the capacity requirements, particularly in hightraffic areas (hot spot areas) that generate the highest volume oftraffic. Network densification—increasing the number of network nodes,thereby bringing them physically closer to the user terminals—is a keyto improving traffic capacity and extending the achievable user-datarates of a wireless communication system.

In addition to straightforward densification of a macro deployment,network densification can be achieved by the deployment of complementarylow-power nodes respectively small cells under the coverage of anexisting macro-node layer. In such a heterogeneous deployment, thelow-power nodes provide very high traffic capacity and very high userthroughput locally, for example in indoor and outdoor hotspot positions.Meanwhile, the macro layer ensures service availability and QoE over theentire coverage area. In other words, the layer containing the low-powernodes can also be referred to as providing local-area access, incontrast to the wide-area-covering macro layer.

The installation of low-power nodes respectively small cells as well asheterogeneous deployments has been possible since the first release ofLTE. In this regard, a number of solutions have been specified in recentreleases of LTE (i.e., Release-10/11). More specifically, these releasesintroduced additional tools to handle inter-layer interference inheterogeneous deployments. In order to further optimize performance andprovide cost/energy-efficient operation, small cells require furtherenhancements and in many cases need to interact with or complementexisting macro cells. Such solutions will be investigated during thefurther evolution of LTE—Release 12 and beyond. In particular furtherenhancements related to low-power nodes and heterogeneous deploymentswill be considered under the umbrella of the new Rel-12 study item (SI)“Study on Small Cell Enhancements for E-UTRA and E-UTRAN”. Some of theseactivities will focus on achieving an even higher degree of interworkingbetween the macro and low-power layers, including different forms ofmacro assistance to the low-power layer and dual-layer connectivity.Dual connectivity implies that the device has simultaneous connectionsto both macro and low-power layers.

Some deployment scenarios assumed in this study item on small cellenhancements will be discussed below. In the following scenarios, thebackhaul technologies categorized as non-ideal backhaul in TR 36.932 areassumed.

Both ideal backhaul (i.e., very high throughput and very low latencybackhaul such as dedicated point-to-point connection using opticalfiber) and non-ideal backhaul (i.e., typical backhaul widely used in themarket such as xDSL, microwave, and other backhauls like relaying)should be studied. The performance-cost trade-off should be taken intoaccount.

A categorization of non-ideal backhaul based on operator inputs islisted in the table below:

Backhaul Latency Priority Technology (One way) Throughput (1 is thehighest) Fiber Access 1 10-30 ms 10M-10 Gbps 1 Fiber Access 2  5-10 ms100-1000 Mbps 2 Fiber Access 3 2-5 ms 50M-10 Gbps 1 DSL Access 15-60 ms10-100 Mbps 1 Cable 25-35 ms 10-100 Mbps 2 Wireless  5-35 ms 10 Mbps-100Mbps 1 Backhaul typical, maybe up to Gbps range

Fiber access which can be used to deploy Remote Radio Heads (RRHs) isnot assumed in this study. HeNBs are not precluded, but notdistinguished from Pico eNBs in terms of deployment scenarios andchallenges even though the transmission power of HeNBs is lower thanthat of Pico eNBs. The following 3 scenarios are considered.

Scenario #1 is illustrated in FIG. 5 and is the deployment scenariowhere macro and small cells on the same carrier frequency(intra-frequency) are connected via a non-ideal backhaul. User aredistributed both for outdoor and indoor.

Scenario #2 is illustrated in FIGS. 6 and 7 and refers to a deploymentscenario where macro and small cells on different carrier frequencies(inter-frequency) are connected via a non-ideal backhaul. User aredistributed both for outdoor and indoor. There are essentially twodifferent scenarios #2, referred herein as 2a and 2b, the differencebeing that in scenario 2b an indoor small cell deployment is considered.

Scenario #3 is illustrated in FIG. 8 and refers to a deployment scenariowhere only small cells on one or more carrier frequencies are connectedvia a non-ideal backhaul link.

Depending on the deployment scenario, different challenges/problemsexist which need to be further investigated. During the study item phasesuch challenges have been identified for the corresponding deploymentscenarios and captured in TS 36.842; more details on thosechallenges/problems can be found there.

In order to resolve the identified challenges which are described insection 5 of TS36.842, the following design goals are taken into accountfor this study in addition to the requirements specified in TR 36.932.

In terms of mobility robustness:

-   -   For UEs in RRC_CONNECTED, Mobility performance achieved by small        cell deployments should be comparable with that of a macro-only        network.    -   In terms of increased signaling load due to frequent handover:    -   Any new solutions should not result in excessive increase of        signaling load towards the Core Network. However, additional        signaling and user plane traffic load caused by small cell        enhancements should also be taken into account.    -   In terms of improving per-user throughput and system capacity:    -   Utilizing radio resources across macro and small cells in order        to achieve per-user throughput and system capacity similar to        ideal backhaul deployments while taking into account QoS        requirements should be targeted.        Dual Connectivity

One promising solution to the problems which are currently underdiscussion in 3GPP RAN working groups is the so-called “dualconnectivity” concept. The term “dual connectivity” is used to refer toan operation where a given UE consumes radio resources provided by atleast two different network nodes connected with a non-ideal backhaul.Essentially, the UE is connected with both a macro cell (macro eNB) andsmall cell (secondary or small eNB). Furthermore, each eNB involved indual connectivity for a UE may assume different roles. Those roles donot necessarily depend on the eNB's power class and can vary among UEs.

Since the study Item is currently at a very early stage, details on thedual connectivity are not decided yet. For example the architecture hasnot been agreed on yet. Therefore, many issues/details, e.g. protocolenhancements, are still open currently. FIG. 9 shows an exemplaryarchitecture for dual connectivity. It should be only understood as onepotential option; the invention is not limited to this specificnetwork/protocol architecture but can be applied generally. Thefollowing assumptions on the architecture are made here:

-   -   Per bearer level decision where to serve each packet, C/U plane        split    -   As an example UE RRC signaling and high QoS data such as VoLTE        can be served by the Macro cell, while best effort data is        offloaded to the small cell.    -   No coupling between bearers, so no common PDCP or RLC required        between the Macro cell and small cell    -   Looser coordination between RAN nodes    -   SeNB has no connection to S-GW, i.e. packets are forwarded by        MeNB    -   Small Cell is transparent to CN.

Regarding the last two bullet points, it should be noted that it's alsopossible that SeNB is connected directly with the S-GW, i.e. S1-U isbetween S-GW and SeNB. Essentially there are three different optionsw.r.t the bearer mapping/splitting:

-   -   Option 1: S1-U also terminates in SeNB; depicted in FIG. 10a    -   Option 2: S1-U terminates in MeNB, no bearer split in RAN;        depicted in FIG. 10b    -   Option 3: S1-U terminates in MeNB, bearer split in RAN; depicted        in FIG. 10c

FIG. 10a-c depict those three options taking the downlink direction forthe U-Plane data as an example. For explanation purpose, option 2 ismainly assumed for this application, and is the basis for FIG. 9 too.

One of the main aims in SCE (Small Cell Enhancement) study item andgenerally in Heterogeneous deployment is that the offloading of UEs tothe small/Pico Cells are maximized which means that more and more UEsare offloaded to small/Pico Cells as well as the time that the UEs stayin the small/Pico Cells (before their connection moving out to a MacroCells) is maximized i.e. the ToS (Time of Stay) maximization is animportant aim in the Small Cell Enhancement and generally inHeterogeneous deployment.

Shortcomings with Heterogeneous Deployment

A state where the UE is known to the network at the Access Stratum levelis generally referred to as Connected Mode. In this respect, ConnectedMode mobility typically indicates when the UE moves from one location tothe other such that the signal condition in the source area is gettingweaker and better in the destination area. The Connected Mode mobilityis realized by means of Handovers. In Heterogeneous deployment scenariosthe source/destination cells are typically of different size/transmitpower.

It has been widely acknowledged that Connected Mode Mobility inHeterogeneous deployment scenarios is not as robust as desired and thathandover failures, HOF, or radio link failures RLF have recently beenput under technical review; for a detailed description it is refer to3GPP TR 36.839: ““Evolved Universal Terrestrial Radio Access (E-UTRA);Mobility enhancements in heterogeneous networks” version 11.1.0,sections 5.2 and 5.4 available at http//www.3gpp.org and incorporatedherein by reference.

The mobility is especially weak when the UE moves out of the Small/Picocell towards the Macro cell. This is mainly so since the radio conditionin the Small/Pico cell's edge is weaker and interfered by the Macrocells transmissions. In these situations it is difficult to successfullyreceive the handover command by the UE.

Some kind of preventive solutions do exist. The preventive solutions inthe state-of-the-art are quite complex and in some cases not completelyclear how these will work; for example:

Repetition of HO CMD message by cells other than the source cell (RRCDiversity) e.g. the handover target cell requires that the UE be able toreceive from 2 different cells [Simultaneous or One by one] and it willbe unattractive and complex since:

-   -   Simultaneous reception requires dual connection capability which        may not be available with all UEs and dual connectivity itself        may not be a desirable solution in some deployment scenarios        (e.g. in Scenario 1 of SCE).    -   One by one (first by source and then by target): In this case,        it is not clear when and how will the UE know, which C-RNTI to        use, which SRB configuration to use etc. when trying to receive        the HO CMD from the Target Cell.

Requiring multiple (interfering) neighbor cells to coordinate/blanktheir transmission when the HO CMD is to be sent/received to/by the UE.However, this alone is not sufficient for non ideal ABS coordinationamong macro cells and larger CRE bias cases.

One of the ways to achieve “prevention” is by sending the HO CMD earlyby the source cell (while the Radio Conditions are sufficiently good);this suffers from:

-   -   Minimizing offloading gains (the UE should stay on the small        cell for as long as possible)    -   Un-necessary handovers/mobility/signaling    -   More HO interruption

Other possible line of solution for improving the Mobility Robustness inheterogeneous deployment could be curative in nature. For example, ifthe mobility/handover fails (e.g. HOF, RLF happens) then how to minimizethe damages (e.g. by faster reestablishments). The curative methods arenot sufficient since they lead to more complexity (in enhancingRe-establishment) and still cause some jitters.

Another problem in Heterogeneous deployment is the UE batteryconsumption in:

-   1) Finding/discovering the small cell layer since these small cells    may not be ubiquitously present everywhere.-   2) Measuring 2 source frequencies while in dual connectivity which    might be un-necessary sometimes.

Improving UE battery life in Heterogeneous deployment is referred to asthe second problem in the remaining text.

SUMMARY OF THE INVENTION

One object of the invention is to provide an improved method forperforming a handoff of a mobile terminal to a target base station. Morespecifically, in performing the handoff the mobile terminal remainsunder the control of the target base station, while at the same timerobustness and reliability of the decision to handoff to the target basestation is improved.

The object is solved by the subject-matter of the independent claims.Advantageous embodiments are subject to the dependent claims.

For the first aspect of the invention, it is assumed that the mobileterminal is configured to receive a handoff command message for thehandoff to the target base station. As part of the handoff, the mobileterminal is to be configured for communication with the target basestation via a target radio cell comprising a downlink carrier and anuplink carrier. The target base station controls configuration of themobile terminal.

Conventionally, a handoff procedure implies that the mobile terminal isconfigured for communication with a source base station via a sourceradio cell. In this respect, the mobile terminal receives the handoffcommand message from the source radio cell via a downlink carrier of thesource radio cell. However, the invention is not limited in thisrespect.

The mobile terminal may also be configured to receive the handoffcommand message from a network entity of different a radio accesstechnology, for instance a WIFI access point, a Bluetooth gateway or aWIMAX router. Irrespective of the radio access technology, it isimportant to note that the invention is performed under the control ofthe target base station. Accordingly, it is the target base stationwhich generates the handoff command message to be received by the mobileterminal.

Upon receipt of a handoff command message, the mobile terminal of theinvention refers to the handoff execution condition additionallyincluded in the handoff command message for information on the executionof the handoff to the target base station. Accordingly, the handoffexecution condition included in the handoff command message can beunderstood as a trigger for the execution of the handoff. In thisrespect, an additional step of evaluating the handoff executioncondition is performed before the handoff to the target message iscarried out.

In more detail, the handoff execution condition includes a time triggerfor the execution of the handoff to the target base station. For thispurpose, the mobile terminal determines, based on the received handoffexecution condition included in the handoff command message, whether ornot the mobile terminal is to trigger execution of the handoff to thetarget base station. Moreover, since the handoff execution condition maydelay or may even prevent the mobile terminal from executing handoff tothe target base station, the handoff execution condition cannot beassumed to result in a one-way-street situation wherein the mobileterminal immediately executes the handoff to the target base station.

In other words, due to the additional determination of whether or notthe handoff execution condition is met, the point in time when themobile terminal receives the handoff command message is separate from(e.g. spaced apart from) the point in time when the mobile terminalexecutes the handoff to the target base station.

This separation is advantageous in view of an optimal timing of thehandoff: The mobile terminal may delay or defer execution of the handoffto the target base station up to a point in time where its connectivity(e.g. to the source radio cell) is lost. In this respect, a handoff tothe target base station may be delayed or deferred up to a point in timewhere reception of the handoff command message would no longer bepossible. Accordingly, the invention strives to prevent from too earlyhandoffs by solving the problem of too late handoffs (i.e. lost handoffcommand messages due to poor radio conditions in the source radio cell).

Further, the additional determination of whether or not the handoffexecution condition is met leaves the mobile terminal with an additionaldegree of freedom for it to determine an ideal timing of the executionof the handoff to the mobile terminal.

However, this additional degree of freedom may be well balanced in thatthe handoff command message includes an indication of a specific handoffexecution condition which is to trigger execution of the handoff to thetarget base station. In other words, by specifying the handoff executioncondition included in the handoff command message, the mobile terminalunder control of the target base station executes handoff in a mannerthat is predictable time-wise to the target bases station.

In more detail, in case the mobile terminal determines that it is totrigger execution of the handoff to the target base station, the mobileterminal executes the handoff to the target base station by (a)performing a random access channel, RACH, procedure with the target basestation to thereby time align the uplink carrier of the target radiocell, and (b) transmitting to the target base station a handoff completemessage. Thereby, the mobile terminal indicates completion of thehandoff to the target base station including indicating completion ofconfiguration of the mobile terminal for communication with the targetbase station via the target radio cell.

According to an embodiment in line with the first aspect of theinvention, a method is suggested for performing a handoff of a mobileterminal to a target base station, wherein the mobile terminal is to beconfigured, under control of the target base station, for communicationwith the target base station via a target radio cell comprising adownlink carrier and an uplink carrier. The mobile terminal receives ahandoff command message for the handoff to the target base stationincluding a handoff execution condition as trigger for executing handoffto the target base station. Then, the mobile terminal determines, basedon the received handoff execution condition included in the handoffcommand message, whether or not the mobile terminal is to triggerexecution of the handoff to the target base station. In case the mobileterminal determines that it is to trigger execution of the handoff tothe target base station, the mobile terminal executes the handoff to thetarget base station (a) by performing a random access channel, RACH,procedure with the target base station to thereby time align the uplinkcarrier of the target radio cell, and (b) by transmitting to the targetbase station a handoff complete message thereby indicating completion ofthe handoff to the target base station including completion ofconfiguration of the mobile terminal for communication with the targetbase station via the target radio cell.

According to a more detailed embodiment of the method for performing thehandoff, the mobile terminal, prior to receiving the handoff commandmessage, is configured for communication with a source base station viaa source radio cell; and the mobile terminal receives the handoffcommand message from the source base station via a downlink carrier ofthe source radio cell; and after receiving the handoff command messagethe mobile terminal detaches from the source radio cell thereby stoppingcommunication with the source base station.

According to another more detailed embodiment of the method forperforming the handoff, in case the mobile terminal does not, within afirst pre-configured period of time, determine that it is to triggerexecution of the handoff to the target base station, the mobile terminaldiscards the received handoff command message.

According to further more detailed embodiment of the method forperforming the handoff, the received handoff command message includes ahandoff validity timer, and the mobile terminal further re-configuresthe first pre-configured period of time based on the handoff validitytimer.

According to yet another more detailed embodiment of the method forperforming the handoff, the handoff execution condition, included in thereceived handoff command message, corresponds to a timer value; and themobile terminal further determines whether or not a timer with the timervalue has expired; wherein, upon expiration of the timer with the timervalue, the mobile terminal is to trigger execution of the handoff to thetarget base station.

According to an even further more detailed embodiment of the method forperforming the handoff, the handoff execution condition, included in thereceived handoff command message, corresponds to an indication totrigger execution of the handoff based on pre-configured thresholdvalues for the signal strength or signal quality of a source and/ortarget radio cell; and the mobile terminal further determines whether ornot the respective signal strength or signal quality of the source radiocell falls below one of the pre-configured threshold values and/or therespective signal strength or signal quality of target radio cell risesabove another of the pre-configured threshold values; wherein, upon therespective signal strength or signal quality having fallen below the oneof the pre-configured threshold values and/or having risen above theother of the pre-configured threshold values, the mobile terminal is totrigger execution of the handoff to the target base station.

According to another more detailed embodiment of the method forperforming the handoff, the handoff execution condition, included in thereceived handoff command message, corresponds to at least one thresholdvalue for the signal strength or signal quality of a source and/ortarget radio cell; and the mobile terminal further determines whether ornot the respective signal strength or signal quality of the source radiocell falls below one of the received threshold value(s) and/or therespective signal strength or signal quality of target radio cell risesabove another of the received threshold value(s); wherein, upon therespective signal strength or signal quality having fallen below one ofthe at least one threshold value(s) and/or having risen above the otherof the at least one threshold value(s), the mobile terminal is totrigger execution of the handoff to the target base station.

According to further more detailed embodiment of the method forperforming the handoff, the threshold value for the signal strengthcorresponds to a threshold value for the reference signal receivedpower, RSRP, to be measured by the mobile terminal based on referencesignals, RSs, transmitted in the source and/or target radio cell; and/oror the threshold value for the signal quality corresponds to a thresholdvalue for the reference signal received quality, RSRQ, to be measured bythe mobile terminal based on reference signals, RSs, transmitted in thesource and/or target radio cell.

According to yet another more detailed embodiment of the method forperforming the handoff, the handoff execution condition, included in thereceived handoff command message, corresponds to an indication totrigger execution of the handoff based on out-of-sync events where achannel quality of a source radio cell falls below a pre-configuredout-of-sync threshold (Qout); and the mobile terminal further determineswhether or not the number of out-of-sync events exceeds thepre-configured number (N310) for the predefined period of time, wherein,in case the number of out-of-sync events exceeds the pre-configurednumber (N310) for the predefined period of time, the mobile terminal isto trigger execution of the handoff to the target base station.

According to an even further more detailed embodiment of the method forperforming the handoff, the handoff execution condition, included in thereceived handoff command message, corresponds to a counter value forout-of-sync events where a channel quality of a source radio cell fallsbelow a pre-configured out-of-sync threshold (Qout); and the determiningmobile terminal further determines whether or not the number ofout-of-sync events exceeds the received counter value (N310-alike) forthe predefined period of time, wherein, in case the number ofout-of-sync events exceeds the received counter value (N310-alike) forthe predefined period of time, the mobile terminal is to triggerexecution of the handoff to the target base station.

According to another more detailed embodiment of the method forperforming the handoff, the handoff command message corresponds to aRRCConnectionReconfiguration message.

According to further more detailed embodiment, the method for performingthe handoff, the handoff complete message corresponds to aRRCConnectionReconfigurationComplete message.

According to yet another more detailed embodiment of the method forperforming the handoff, in case the mobile terminal determines, within asecond pre-configured period of time, that it is to trigger execution ofthe handoff to the target base station, the mobile terminal executes thehandoff to the target base station including (a) performing acontention-free RACH procedure based on a random access preambleincluded in the handoff command message; and in case the mobile terminaldetermines, after expiry of the second pre-configured period of time,that it is to trigger execution of the handoff to the target basestation, the mobile terminal executes the handoff to the target basestation including (a) performing a contention-based RACH procedure basedon a random access preamble randomly selected by the mobile terminal.

According to an even further more detailed embodiment of the method forperforming the handoff, the received handoff command message includes aRACH validity timer, and the mobile terminal further re-configures thesecond pre-configured period of time based on the RACH validity timer.

According to another embodiment in line with the first aspect of theinvention, a mobile terminal is suggested for performing a handoff of amobile terminal to a target base station, wherein the mobile terminal isto be configured, under control of the target base station, forcommunication with the target base station via a target radio cellcomprising a downlink carrier and an uplink carrier. A receiving circuitof the mobile terminal is configured to receive a handoff commandmessage for the handoff to the target base station including a handoffexecution condition as trigger for executing handoff to the target basestation. A processor of the mobile terminal is configured to determine,based on the received handoff execution condition included in thehandoff command message, whether or not the mobile terminal is totrigger execution of the handoff to the target base station. In case theprocessor of the mobile terminal determines that the mobile terminal isto trigger execution of the handoff to the target base station, themobile terminal executes the handoff to the target base station (a) byperforming a random access channel, RACH, procedure with the target basestation to thereby time align the uplink carrier of the target radiocell, and (b) by transmitting to the target base station a handoffcomplete message thereby indicating completion of the handoff to thetarget base station including completion of configuration of the mobileterminal for communication with the target base station via the targetradio cell.

According to a more detailed embodiment, the mobile terminal, prior toreceiving the handoff command message, is configured for communicationwith a source base station via a source radio cell; and the receivingcircuit is configured to receive the handoff command message from thesource base station via a downlink carrier of the source radio cell;and, after receiving the handoff command message, the mobile terminal isconfigured to detach from the source radio cell whereby communicationwith the source base station is stopped.

According to another more detailed embodiment of the mobile terminal, incase the processor of the mobile terminal does not, within a firstpre-configured period of time, determine that the mobile terminal is totrigger execution of the handoff to the target base station, the mobileterminal is configured to discard the received handoff command message.

According to further more detailed embodiment of the mobile terminal,the received handoff command message includes a handoff validity timer,and the processor of the mobile terminal is configured to re-configurethe first pre-configured period of time based on the handoff validitytimer.

According to yet another more detailed embodiment of the mobileterminal, the handoff execution condition, included in the receivedhandoff command message, corresponds to a timer value; and the processorof the mobile terminal is configured to determine whether or not a timerwith the timer value has expired; wherein, upon expiration of the timerwith the timer value, the mobile terminal is to trigger execution of thehandoff to the target base station.

According to an even further more detailed embodiment of the mobileterminal, the handoff execution condition, included in the receivedhandoff command message, corresponds to an indication to triggerexecution of the handoff based on pre-configured threshold values forthe signal strength or signal quality of a source and/or target radiocell; and the processor of the mobile terminal is configured todetermine whether or not the respective signal strength or signalquality of the source radio cell falls below one of the pre-configuredthreshold values and/or the respective signal strength or signal qualityof target radio cell rises above another of the pre-configured thresholdvalues; wherein, upon the respective signal strength or signal qualityhaving fallen below the one of the pre-configured threshold valuesand/or having risen above the other of the pre-configured thresholdvalues, the mobile terminal is to trigger execution of the handoff tothe target base station.

According to another more detailed embodiment of the mobile terminal,the handoff execution condition, included in the received handoffcommand message, corresponds to at least one threshold value for thesignal strength or signal quality of a source and/or target radio cell;and the processor of the mobile terminal is configured to determinewhether or not the respective signal strength or signal quality of thesource radio cell falls below one of the received threshold value(s)and/or the respective signal strength or signal quality of target radiocell rises above another of the received threshold value(s); wherein,upon the respective signal strength or signal quality having fallenbelow one of the at least one threshold value(s) and/or having risenabove the other of the at least one threshold value(s), the mobileterminal is to trigger execution of the handoff to the target basestation.

According to further more detailed embodiment of the mobile terminal,the threshold value for the signal strength corresponds to a thresholdvalue for the reference signal received power, RSRP, to be measured bythe mobile terminal based on reference signals, RSs, transmitted in thesource and/or target radio cell; and/or the threshold value for thesignal quality corresponds to a threshold value for the reference signalreceived quality, RSRQ, to be measured by the mobile terminal based onreference signals, RSs, transmitted in the source and/or target radiocell.

According to yet another more detailed embodiment of the mobileterminal, the handoff execution condition, included in the receivedhandoff command message, corresponds to an indication to triggerexecution of the handoff based on out-of-sync events where a channelquality of a source radio cell falls below a pre-configured out-of-syncthreshold (Qout); and the processor of the mobile terminal is configuredto determine whether or not the number of out-of-sync events exceeds thepre-configured number (N310) for the predefined period of time, wherein,in case the number of out-of-sync events exceeds the pre-configurednumber (N310) for the predefined period of time, the mobile terminal isto trigger execution of the handoff to the target base station.

According to an even further more detailed embodiment of the mobileterminal, the handoff execution condition, included in the receivedhandoff command message, corresponds to a counter value for out-of-syncevents where a channel quality of a source radio cell falls below apre-configured out-of-sync threshold (Qout); and the processor of themobile terminal is configured to determine whether or not the number ofout-of-sync events exceeds the received counter value (N310-alike) forthe predefined period of time, wherein, in case the number ofout-of-sync events exceeds the received counter value (N310-alike) forthe predefined period of time, the mobile terminal is to triggerexecution of the handoff to the target base station.

According to another more detailed embodiment of the mobile terminal,the handoff command message corresponds to aRRCConnectionReconfiguration message.

According to further more detailed embodiment of the mobile terminal,the handoff complete message corresponds to aRRCConnectionReconfigurationComplete message.

According to yet another more detailed embodiment of the mobileterminal, in case the processor determines, within a secondpre-configured period of time, that the mobile terminal is to triggerexecution of the handoff to the target base station, the mobile terminalexecutes the handoff to the target base station including performing acontention-free RACH procedure based on a random access preambleincluded in the handoff command message; and in case the processordetermines, after expiry of the second pre-configured period of time,that the mobile terminal is to trigger execution of the handoff to thetarget base station, the mobile terminal executes the handoff to thetarget base station including performing a contention-based RACHprocedure based on a random access preamble randomly selected by themobile terminal.

According to an even further more detailed embodiment of the mobileterminal, the received handoff command message includes a RACH validitytimer, and the processor of the mobile terminal is configured tore-configure the second pre-configured period of time based on the RACHvalidity timer.

According to a further embodiment in line with the first aspect of theinvention, a computer readable medium is suggested which is storinginstructions that, when executed by a processor of a mobile terminal,cause the mobile terminal to perform a handoff to a target base station,wherein the mobile terminal is to be configured, under control of thetarget base station, for communication with the target base station viaa target radio cell comprising a downlink carrier and an uplink carrier,by: receiving a handoff command message for the handoff to the targetbase station including a handoff execution condition as trigger forexecuting handoff to the target base station; and determining, based onthe received handoff execution condition included in the handoff commandmessage, whether or not the mobile terminal is to trigger execution ofthe handoff to the target base station; wherein, in case the mobileterminal determines that it is to trigger execution of the handoff tothe target base station, the mobile terminal executes the handoff to thetarget base station (a) by performing a random access channel, RACH,procedure with the target base station to thereby time align the uplinkcarrier of the target radio cell, and (b) by transmitting to the targetbase station a handoff complete message thereby indicating completion ofthe handoff to the target base station including completion ofconfiguration of the mobile terminal for communication with the targetbase station via the target radio cell.

According to a different embodiment of the invention, a method isproposed for configuring measurement cycles of a mobile terminal beingconnected via a macro radio cell to a master base station and via asmall radio cell to a secondary base station, the macro and the smallradio cell being configured for a same frequency. The master basestation determines a geographical distance between the master basestation and the secondary base station. Then, the master base stationcompares the determined geographical distance to a pre-configureddistance threshold. Thereafter, the master base station generates ameasurement control message for the mobile terminal including a firstmeasurement object with a measurement cycle for the macro radio cell anda second measurement object with a measurement cycle for the small radiocell; and the master base station transmits the measurement controlmessage to the mobile terminal for configuring measurement cycles of themobile terminal; wherein the measurement cycle within the transmittedfirst measurement object for the macro radio cell is set based on aresult of the comparison of the determined geographical distance to thepre-configured distance threshold.

According to a more detailed embodiment of the method for configuringmeasurement cycles, in case the result of the comparison indicates thatthe determined geographical distance is smaller than the pre-configureddistance threshold, the master base station further sets the measurementcycle within the transmitted first measurement object for the macroradio cell to a value which indicates measurement relaxation to themobile terminal.

According to another more detailed embodiment of the method forconfiguring measurement cycles, in case the result of the comparisonindicates that the determined geographical distance is smaller than thepre-configured distance threshold, the master base station further setsthe measurement cycle within the transmitted first measurement objectfor the macro radio cell to a value which corresponds to more than 5measurement cycles of the small radio cell.

According to further more detailed embodiment of the method forconfiguring measurement cycles, in case the result of the comparisonindicates that the determined geographical distance is smaller than thepre-configured distance threshold, the master base station further setsthe measurement cycle within the transmitted first measurement objectfor the macro radio cell to a value which indicating deactivation of themeasurements.

According to yet another more detailed embodiment of the method forconfiguring measurement cycles, the master base station looks up thegeographical position of the secondary base station from a locationinformation table stored in the master base station.

According to an even further more detailed embodiment of the method forconfiguring measurement cycles, the master base station receives fromthe secondary base station location information indicating thegeographical position thereof.

According to another more detailed embodiment of the method forconfiguring measurement cycles, the master base station measures asignal strength of reference signals that are transmitted by thesecondary base station via the small radio cell.

According to further more detailed embodiment of the method forconfiguring measurement cycles, the master base station receives atleast one measurement report from the mobile terminal indicating asignal strength of reference signals that are transmitted by thesecondary base station via the small radio cell and indicating a signalstrength of reference signals that are transmitted by the master basestation via the macro radio cell.

According to yet another more detailed embodiment of the method forconfiguring measurement cycles, the master base station measures around-trip-time of information transmitted to and received from thesecondary base station.

According to another different embodiment of the invention, a masterbase station is proposed for configuring measurement cycles of a mobileterminal being connected via a macro radio cell to the master basestation and via a small radio cell to a secondary base station, themacro and the small radio cell being configured for a same frequency. Aprocessor of the master base station is configured to determine ageographical distance between the master base station and the secondarybase station; and the processor of the master base station is furtherconfigured to compare by the master base station the determinedgeographical distance to a pre-configured distance threshold.

A transmitting circuit of the master base station is configured togenerate a measurement control message for the mobile terminal includinga first measurement object with a measurement cycle for the macro radiocell and a second measurement object with a measurement cycle for thesmall radio cell; and the transmitting circuit of the master basestation is further configured to transmit the measurement controlmessage to the mobile terminal for configuring measurement cycles of themobile terminal. The measurement cycle within the transmitted firstmeasurement object for the macro radio cell is set by the master basestation based on a result of the comparison of the determinedgeographical distance to the pre-configured distance threshold.

According to a more detailed embodiment of the master base station, incase the result of the comparison indicates that the determinedgeographical distance is smaller than the pre-configured distancethreshold, the transmitting circuit of the master base station isconfigured to set the measurement cycle within the transmitted firstmeasurement object for the macro radio cell to a value which indicatesmeasurement relaxation to the mobile terminal.

According to another more detailed embodiment of the master basestation, in case the result of the comparison indicates that thedetermined geographical distance is smaller than the pre-configureddistance threshold, the transmitting circuit of the master base stationis configured to set the measurement cycle within the transmitted firstmeasurement object for the macro radio cell to a value which correspondsto more than 5 measurement cycles of the small radio cell.

According to further more detailed embodiment of the master basestation, in case the result of the comparison indicates that thedetermined geographical distance is smaller than the pre-configureddistance threshold, the transmitting circuit of the master base stationis configured to set the measurement cycle within the transmitted firstmeasurement object for the macro radio cell to a value which indicatingdeactivation of the measurements.

According to yet another more detailed embodiment, the processor of themaster base station is configured to look up the geographical positionof the secondary base station from a location information table storedin the master base station.

According to an even further more detailed embodiment, the processor ofthe master base station is configured to receive from the secondary basestation location information indicating the geographical positionthereof.

According to another more detailed embodiment, the processor of themaster base station is configured to measure a signal strength ofreference signals that are transmitted by the secondary base station viathe small radio cell.

According to further more detailed embodiment, a receiving circuit ofthe master base station is configured to receive at least onemeasurement report from the mobile terminal indicating a signal strengthof reference signals that are transmitted by the secondary base stationvia the small radio cell and indicating a signal strength of referencesignals that are transmitted by the master base station via the macroradio cell.

According to yet another more detailed embodiment, a receiving circuitof the master base station is configured to measure station around-trip-time of information transmitted to and received from thesecondary base station.

According to a further different embodiment of the invention, a computerreadable medium is proposed which is storing instructions that, whenexecuted by a processor of a master base station, cause the master basestation to configuring measurement cycles of a mobile terminal beingconnected via a macro radio cell to a master base station and via asmall radio cell to a secondary base station, the macro and the smallradio cell being configured for a same frequency, by: determining ageographical distance between the master base station and the secondarybase station; comparing the determined geographical distance to apre-configured distance threshold; generating a measurement controlmessage for the mobile terminal including a first measurement objectwith a measurement cycle for the macro radio cell and a secondmeasurement object with a measurement cycle for the small radio cell;and transmitting the measurement control message to the mobile terminalfor configuring measurement cycles of the mobile terminal; wherein themeasurement cycle within the transmitted first measurement object forthe macro radio cell is set based on a result of the comparison of thedetermined geographical distance to the pre-configured distancethreshold.

BRIEF DESCRIPTION OF THE FIGURES

In the following the invention is described in more detail withreference to the attached figures and drawings.

FIG. 1 shows an exemplary architecture of a 3GPP LTE system;

FIG. 2 shows an exemplary overview of the overall E-UTRAN architectureof 3GPP LTE;

FIG. 3 shows a sequence diagram of an Intra-MME/Serving Gateway Handoverprocedure as defined in 3GPP LTE Release 8/9;

FIG. 4 illustrates an RRCConnectionReconfiguration message as defined in3GPP LTE Release 8/9 to be used as Handover command message in Handoverprocedures;

FIG. 5 illustrates a deployment scenario for small cell enhancement,where macro and small cells are on the same carrier frequency;

FIGS. 6 and 7 illustrate further deployment scenarios for small cellenhancement where macro and small cells are on different carrierfrequencies, the small cell being respectively outdoor and indoor;

FIG. 8 illustrates a further deployment scenario for small cellenhancement with only small cells;

FIG. 9 gives an overview of the communication system architecture fordual connectivity with macro and small eNBs connected to the corenetwork, where the S1-U interface terminates in the Macro eNB and nobearer splitting is done in RAN;

FIGS. 10a-c illustrate the different options for having two separate EPSbearers between the SGW and the UE;

FIG. 11 exemplifies an improved handover procedure according to thefirst embodiment of the invention;

FIG. 12 illustrates a handover command message including a handoverexecution condition according to the first embodiment of the invention;

FIGS. 13a-c show different implementations of handover command messagesaccording to the first embodiment of the invention;

FIG. 14 illustrates a third implementation of the handover procedureaccording to the first embodiment in more detail;

FIG. 15 shows a sequence diagram of the second embodiment of theinvention; and

FIG. 16 illustrates an exemplary deployment scenario considered inconnection with the second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following paragraphs will describe various embodiments of theinvention. For exemplary purposes only, most of the embodiments areoutlined in relation to a radio access scheme according to 3GPP LTE(Release 8/9) and LTE-A (Release 10/11) mobile communication systems,partly discussed in the Technical Background section above. It should benoted that the invention may be advantageously used, for example, in amobile communication system such as 3GPP LTE-A (Release 10/11/12)communication systems as described in the Technical Background sectionabove, but the invention is not limited to its use in this particularexemplary communication networks.

In the context of the invention, a “mobile terminal” or “mobile node” isto be understood as a physical entity within a wireless communicationnetwork. One node may have several functional entities. A functionalentity refers to a software or hardware module that implements and/oroffers a predetermined set of functions to other functional entities ofa node or the network. Nodes may have one or more interfaces that attachthe node to a communication facility or medium over which nodes cancommunicate. Similarly, a network entity may have a logical interfaceattaching the functional entity to a communication facility or mediumover which it may communicate with other functional entities orcorrespondent nodes.

Further, the term “master base station” used throughout the descriptionof the invention is to be construed in the sense of the terms macro basestation, or master/macro eNB conventionally found in the field of dualconnectivity of 3GPP LTE-A. Similarly, the term “secondary base station”is to be understood in the same sense as the terms slave base station,or secondary/slave eNB also used in connection with dual connectivity.

With respect to the terms “handoff” or “handoff procedure” it shall beemphasized that the invention intends to avoid any implications that areinherent to the terms “handover” or “handover procedure” in wirelesscommunication. Moreover, the term “handoff” is to be understood asreferring to any attempt by a mobile terminal to set up communicationwith a target base station via a target radio cell comprising a downlinkcarrier and an uplink carrier.

Nevertheless, it shall be pointed out that the terms “handoff” or“handoff procedure” will be referred to as “handover” or as “handoverprocedure” in order to allow for a consistent explanation in connectionwith the terminology employed by 3GPP LTE. In other words, although theterm “handoff” is to be construed in a broad sense, the detaileddescription outlines the invention as various implementations referringto the term “handover” generally used in 3GPP LTE. In that sense, the“handoff command message” and the “handoff complete message” are linkedto the “handover command message” and “handover complete message”defined in the background art.

The term “condition” or more precisely “handoff execution condition” hasbeen used throughout the description as a name for an additional datafield of the handoff command message. The handoff execution conditionrefers to a condition to be evaluated by the mobile terminal. In thisrespect, the handoff execution condition may indicate that the mobileterminal is to evaluate the indicated condition based on apre-configured value. Alternatively, the handoff execution condition maybe an actual value based on which the indicated condition is to beevaluated by the mobile terminal.

Additionally, in the description the term “geographical distance” hasbeen employed to denote a coverage relationship between a master basestation and a secondary base station. In other words, in case thesecondary and the master base station provide for a same or a similarcoverage area, both base stations are referred to as being located at asmall geographical distance from each other. Similarly, in case themaster and the secondary base station provide for substantiallydifferent coverage areas, both base stations are referred to as beinglocated at a large geographical distance from each other.

Nevertheless, it shall be pointed out that the term “geographicaldistance” is not the only term to reflect this coverage relationshipbetween the master and the secondary base station, but equally the term“path loss” can be used. For example, in case of a small “path loss”between the master and the secondary base station, the two base stationsprovide for a same or similar coverage area, and hence, are referred toas being located at a small geographical distance from each other.Similarly, in case of a high “path loss” between the master and thesecondary base station, the two base stations provide for substantiallydifferent coverage areas, and hence, are referred to as being located ata large geographical distance from each other. Conseqently, the twoterms “geographical distance” and “path loss” shall be understood asequivalents in the context of the invention.

In the following, several embodiments of the invention will be explainedin detail. These embodiments are described as implementations for use inconnection with and/or for enhancement of functionality specified in3GPP LTE and/or LTE-A. In this respect, the terminology of 3GPP LTEand/or LTE-A is employed throughout the description. Further, exemplaryconfigurations are explored to detail the full breadth of the invention.

The explanations should not be understood as limiting the invention, butas a mere example of the invention's embodiments to better understandthe invention. A skilled person should be aware that the generalprinciples of the invention as laid out in the claims can be applied todifferent scenarios and in ways that are not explicitly describedherein. Correspondingly, the following scenarios assumed for explanatorypurposes of the various embodiments shall not limit the invention assuch.

First Embodiment

Referring now to the first embodiment of the invention, variousimplementations of an improved handover procedure are to be discussed inconnection with FIGS. 11 to 14. Specifically, FIG. 11 illustrates asequence diagram of the handover procedure to a target base station tobe performed by a mobile terminal.

In the context of the first embodiment, it is assumed that the mobileterminal is initially configured for communication with a source basestation via a source radio cell. The mobile terminal is in the RRCCONNECTED state with respect to the source radio cell. The mobileterminal then performs the handover procedure for it to be configuredfor communication with the target base station via a target radio cell.As a result of successful completion of the handover procedure, themobile terminal detaches from the source radio cell, and remains in theRRC CONENCTED state with respect to the target radio cell.

The handover procedure is performed under control of the target basestation. In particular, the target base station generates a message forthe mobile terminal to perform the handover (i.e. handover commandmessage). Subsequently, the handover command message for the mobileterminal to perform handover is forwarded to the mobile terminal by thesource base station. Upon receipt of the handover command message (cf.message 7 in FIG. 11), the mobile terminal performs the handoverprocedure. After successful completion of the handover, the mobileterminal indicates same by transmitting a handover complete message tothe target base station (cf. message 11 in FIG. 11).

According to an exemplary implementation, the handover command messageis an RRC message, namely the RRCConnectionReconfiguration message. Thehandover command message originates from the target base station withinformation required for the mobile terminal to establish a connectionthereto, namely the mobilityControlInformation. According to anotherexemplary implementation, the handover complete message is also an RRCmessage, namely the RRCConnectionReconfigurationComplete message.

According to the first embodiment, the handover command message isenhanced to additionally include information termed handover executioncondition. The handover execution condition refers to information basedon which the mobile terminal determines whether or not a handover is tobe performed to the target base station. Accordingly, handover executioncondition included in the handover command message can be understood asa trigger for the execution of the handover. According to the exemplaryimplementation, the RRCConnectionReconfiguration message including the(handover) execution condition is illustrated in FIG. 12.

In this respect, an additional step of evaluating the handover executioncondition is performed by the mobile terminal before the handover to thetarget message is carried out. In case the mobile terminal determinesthat the handover execution condition is fulfilled, the mobile terminalproceeds with triggering execution of the handover to the target basestation. Similarly, in case the mobile terminal determines that thehandover execution condition is not fulfilled, the mobile terminalproceeds with discarding the received handover command message.

The handover execution condition may have different implementations aswill become apparent from the respective description below. Irrespectivethereof, it is inherent to all implementations of the handover executioncondition that the mobile terminal is provided with informationindicating what determination is to be carried out by the mobileterminal and when (i.e. under which condition) it is to triggerexecution of the handover to the target base station.

Accordingly, in view of this definition of the handover executioncondition included in the handover command message it may be appreciatedthat the target base station remains in control of the handoverprocedure. Moreover, the determination of the mobile terminal based onthe handover execution condition merely assists in the target basestation finding an advantageous point in time for the execution of thehandover.

Specifically, in carrying out the determination based on the handoverexecution condition, the mobile terminal may delay or may even preventexecution of the handover to the target base station. In this respect,the handover command message including the handover execution conditioncannot be understood as resulting in a one-way-street situation whereinthe mobile terminal immediately executes the handover to the target basestation.

Advantageously, due to the mobile terminal additionally determiningwhether or not the indicated handover execution condition is met, thepoint in time when the mobile terminal receives the handover commandmessage is separate from (e.g. spaced apart from) the point in time whenthe mobile terminal may execute the handover to the target base station.

This separation is beneficial in view of an optimal timing of thehandover: The mobile terminal may delay/defer execution of the handoverto the target base station up to a point in time where its connectivityhas reduced (e.g. up to the border of the source radio cell). In thisrespect, a handover to the target base station may be delayed/deferredup to a point in time where reception of the handover command messagewould no longer be possible. Accordingly, the invention strives toprevent from too early handovers by solving the problem of too latehandovers (i.e. lost handover command messages due to poor radioconditions in the source radio cell).

Further, the additional determination of whether or not the handoverexecution condition is met leaves the mobile terminal with an additionaldegree of freedom for it to determine an ideal timing of the executionof the handover to the mobile terminal.

However, this additional degree of freedom may be well balanced in thesense that the handover command message includes an indication of aspecific handover execution condition which is to trigger execution ofthe handover to the target base station. In other words, by specifyingthe handover execution condition included in the handover commandmessage, the mobile terminal under control of the target base executeshandover in a manner that is predictable time-wise to the target basestation.

According to an exemplary implementation, the mobile terminal isconfigured to only trigger execution of the handover to the target basestation within a first pre-configured period of time. In other words, incase the mobile terminal does not, within the first pre-configuredperiod of time, determine that it is to trigger execution of thehandover to the target base station, the mobile terminal discards thereceived handover command message. This exemplary implementation furtherimproves predictability to the target base station in the sense that thetarget base station knows the time period during which a handover canoccur.

According to a modification of the above noted exemplary implementation,the handover command message includes a handover validity timer based onwhich the first pre-configured period of time may be set. Moreover, uponreception of a handover command message including the handover validitytimer by the mobile terminal, it is adapted to re-configure the firstpre-configured period of time based on the received handover validitytimer.

Further, in case the mobile terminal determines that it is to triggerexecution of the handover to the target base station, the mobileterminal executes the handover to the target base station.

The execution of the handover to the target base station includes (a)performing a random access channel, RACH, procedure (cf. messages 9 and10 in FIG. 11) with the target base station to thereby time align theuplink carrier of the target radio cell. The timing of the uplinkcarrier is aligned by the mobile terminal based on a timing advance, TA,value received from the target access point. Further, the execution ofthe handover includes (b) transmitting to the target base station ahandover complete message (cf. message 11 in FIG. 11). Thereby thecompletion of the handover to the target base station is indicated bythe mobile terminal. Consequently, the configuration of the mobileterminal for communication with the target base station via the targetradio cell is completed.

In summary, the first embodiment provides for the following advantages:

-   -   The mobile terminal's reception of handover command message from        the source radio cell can be guaranteed since transmission of        this message under bad radio link condition is no longer        necessary.    -   The offloading gain (i.e. Time of Stay of the mobile terminal in        the source radio cell) increases.    -   Radio link failures, RLF (and subsequently radio link        re-establishment) is avoided and therefore RRC Connection        re-establishment time corresponding to a substantial        interruption of radio connectivity is avoided.    -   New mobile terminal's behavior (for monitoring target cell        before handover) requiring dual connectivity is avoided        (therefore, issues like target C-RNTI allocation/PDCCH        monitoring is avoided).    -   No resource restriction/almost blank subframe, ABS.        coordination/handover protection from target radio cell is        required.    -   Dedicated random access preamble reservation for the mobile        terminal upon handover to the target radio cell is not needed;        insteadInstead, any combination of dedicated random access        preamble reservation (for a pre-configured period of time) and        thereafter contention based RACH is suggested for handover by        the mobile terminal to the target radio cell.        First Implementation

According to a first implementation of the first embodiment, thehandover execution condition corresponds to a timer value. The timervalue is included in the handover command message as handover executioncondition and received as same by the mobile terminal. Since the targetbase station generates the handover command message, the timer value isknown not only to the mobile terminal but also to the target basestation. An exemplary realization of the handover command messageaccording to the first implementation is shown in FIG. 13 a.

In response to the receipt of the timer value included in the handovercommand message as handover execution condition, the mobile terminaldetermines whether or not a timer of the received timer value hasexpired. Upon expiration of the timer, the mobile terminal is to triggerexecution of the handover to the target base station.

As an implementation constraint, the timer value of this firstimplementation has to be smaller than the first pre-configured period oftime after which the handover command message is discarded. Similarly,the timer value of this first implementation has also to be smaller thana potentially received handover validity timer also determining thefirst pre-configured period of time after which the handover commandmessage is discarded. Otherwise, the mobile terminal would neverdetermine that it is to trigger execution of the handover to the targetbase station.

Second Implementation

According to a second implementation of the first embodiment, thehandover execution condition corresponds to at least one threshold valuefor either a signal strength or a signal quality of the source and/ortarget radio cell. This second implementation makes use of physicallayer measurements performed by the mobile terminal. The thresholdvalue(s) of the signal strength or signal quality of the source and/ortarget radio cell is (are) included in the handover command message ashandover execution condition and are received as same by the mobileterminal. Since the target base station generates the handover commandmessage, the threshold value(s) is (are) known not only to the mobileterminal but also to the target base station. An exemplary realizationof the handover command message according to the second implementationis shown in FIG. 13 b.

Depending on which threshold value(s) are included and hence received inthe handover command message as handover execution condition, threedifferent behaviors can be distinguished:

In case a threshold value for a signal strength or signal quality of thesource radio cell is received by the mobile terminal as handoverexecution condition, the mobile terminal determines whether or not therespective signal strength or signal quality of the source radio cellfalls below the received threshold value. Upon the respective signalstrength or signal quality of the source radio cell having fallen belowthe received threshold value, the mobile terminal is to triggerexecution of the handover to the target base station.

In case a threshold value for a signal strength or signal quality of thetarget radio cell is received by the mobile terminal as handoverexecution condition, the mobile terminal determines whether or not therespective signal strength or signal quality of the target radio cellrises above the received threshold value. Upon the respective signalstrength or signal quality of the target radio cell having risen abovethe received threshold value, the mobile terminal is to triggerexecution of the handover to the target base station.

Further, in case threshold values for a signal strength or signalquality of the source radio cell and the target radio cell are receivedby the mobile terminal as handover execution condition, the mobileterminal determines whether or not the respective signal strength orsignal quality of the source radio cell falls below the receivedthreshold value and, at the same time, whether or not the respectivesignal strength or signal quality of the target radio cell rises abovethe received threshold value. Upon the respective signal strength orsignal quality of the source radio cell having fallen below and therespective signal strength or signal quality of the target radio cellhaving risen above the received threshold values, the mobile terminal isto trigger execution of the handover to the target base station.

According to an exemplary realization, the mobile terminal is todetermine whether or not the signal strength corresponds to a thresholdvalue for the reference signal received power, RSRP, to be measured bythe mobile terminal based on reference signals, RSs, transmitted in thesource and/or target radio cell. Alternatively, the mobile terminal isto determine whether or not the signal quality corresponds to athreshold value for the reference signal received quality, RSRQ, to bemeasured by the mobile terminal based on reference signals, RSs,transmitted in the source and/or target radio cell.

Third Implementation

According to a third implementation of the first embodiment, thehandover execution condition corresponds to a counter value forout-of-sync events where a channel quality of the source radio cellfalls below a preconfigured out-of-sync threshold. This thirdimplementation makes use of a radio link, RL, monitoring processincluded in the mobile terminal. An exemplary realization of thehandover command message according to the third implementation is shownin FIG. 13 c.

In response to the receipt of a counter value for out-of-sync eventsincluded in the handover command message as handover executioncondition, the mobile terminal determines whether or not the number ofout-of-sync events which are reported by the RL monitoring process for apredefined period of time exceeds the received counter value. Upon thenumber of reported out-of-sync events exceeding the received countervalue for the predefined period of time, the mobile terminal is totrigger execution of the handover to the target base station. Theseparation between handover procedure and RL monitoring process isexemplarily detailed in FIG. 14.

In summary, the above three implementations of the first embodiment allenable the mobile terminal to determine whether or not it is to executehandover to the target base station at a point in time which isdifferent (i.e. later) than the point in time when the mobile terminalreceives the handover command message. In this respect, in all threeimplementations the handover to the target base station may bedelayed/deferred up to a point in time where reception of the handovercommand message would no longer be possible.

Exemplarily, the handover command message may be implemented as an RRCmessage, namely the RRCConnectionReconfiguration message, which isdefined as follows:

RRCConnectionReconfiguration-r8-IEs ::= SEQUENCE {  measConfigMeasConfig  OPTIONAL, -- Need ON  mobilityControlInfoMobilityControlInfo  OPTIONAL, -- Cond HO  dedicatedInfoNASList SEQUENCE(SIZE(1..maxDRB)) OF DedicatedInfoNAS  OPTIONAL, -- Cond nonHO radioResourceConfigDedicated RadioResourceConfigDedicated  OPTIONAL, --Cond HO-toEUTRA  securityConfigHO SecurityConfigHO  OPTIONAL, -- Cond HO nonCriticalExtension RRCConnectionReconfiguration-v890-IEs  OPTIONAL --Need OP }

Further, the handover execution condition included in the handovercommand message may be implemented as part of themobilityControlInformation, as specified in the following:

MobilityControlInfo ::= SEQUENCE {  targetPhysCellId  PhysCellId, carrierFreq  CarrierFreqEUTRA OPTIONAL, -- Cond HO-toEUTRA2 carrierBandwidth  CarrierBandwidthEUTRA OPTIONAL, -- Cond HO-toEUTRA additionalSpectrumEmission  AdditionalSpectrumEmission OPTIONAL, --Cond HO-toEUTRA  t304 ENUMERATED {  ms50, ms100, ms150, ms200, ms500, ms1000,ms2000, spare1},  newUE-Identity  C-RNTI, radioResourceConfigCommon  RadioResourceConfigCommon, rach-ConfigDedicated  RACH-ConfigDedicated OPTIONAL, -- Need OP  ..., [[ carrierFreq-v9e0  CarrierFreqEUTRA-v9e0 OPTIONAL -- Need ON  ]],  [[drb-ContinueROHC-r11  ENUMERATED {true} OPTIONAL -- Cond HO  ]] executionConditionValidityTimer ENUMERATED {  ms0, ms100, ms500,ms1000,  ms2000, ms4000, ms5000, spare1}, (OPTIONAL), executionCondition  CHOICE} timeToExecute  ENUMERATED {ms0, ms100,ms500,  ms1000,ms2000, ms4000, ms5000,  spare1}, (OPTIONAL),radioCondition  SEQUENCE {  sourceCellQuality  ReportConfigEUTRA,(OPTIONAL),  targetCellQuality  ReportConfigEUTRA, (OPTIONAL),  },(OPTIONAL), n310-r9  ENUMERATED {n1, n2, n3, n4, n6,  n8, n10, n20}(OPTIONAL),  } (OPTIONAL)

Accordingly, the mobilityControlInformation included in theRRCConnectionReconfiguration message includes three fields, i.e.timeToExecute, radioCondition, and n310-r9, among which, upon generationof the message, the target base station chooses one field to implementin the message to the mobile terminal.

The data field named timeToExecute may include the timer value asdefined in the first implementation. The data field(s) namedradioCondition including sourceCellQuality and/or targetCellQuality mayinclude respective threshold value(s) as defined in the secondimplementation. The data field named n310-r9 may include the countervalue for out-of-sync events as defined in the third implementation.

Further Implementation

According to a further implementation of the first embodiment, themobile terminal varies the random access channel, RACH, procedure to beperformed as part of execution of the handover to the target basestation based on the point in time when the execution of the handover istriggered. Specifically, the mobile terminal distinguishes between anexecution of the handover that is triggered before or after a secondpre-configured period of time.

As commonly known, in 3GPP LTE there are two different RACH proceduresdefined, namely a contention-free RACH procedure and a contention-basedRACH procedure. In the two RACH procedures the mobile terminal utilizesrandom access preambles from different groups for time aligning theuplink carrier of the (e.g. target) radio cell.

In the contention-free RACH procedure, the mobile terminal is assigned aunique random access preamble from a first group and can later therebybe identified by the (e.g. target) base station. In this respect, the(e.g. target) base station allocates the random access preamble from thefirst group to the mobile terminal for it to subsequently perform thecontention-free RACH procedure. Conventionally, the allocated randomaccess preamble is signaled to the mobile terminal as part of thehandover command message.

In the contention-based RACH procedure, the mobile terminal randomlyselects a random access preamble from a second group (which is distinctfrom the first group). As there may be more than one mobile terminalrandomly selecting a same random access preamble, contention can occurand a lengthy mechanism of contention-resolution has to be carried outto enable the mobile terminal time aligning the uplink carrier and toreceive the grant to send further UL signaling/data.

In this further implementation, in case the mobile terminal determines,within a second pre-configured period of time, that it is to triggerexecution of the handover to the target base station, the mobileterminal executes the handover to the target base station includingperforming a contention-free RACH procedure based on a random accesspreamble included in the handover command message.

Alternatively, in this further implementation, in case the mobileterminal determines, after expiry of the second pre-configured period oftime, that it is to trigger execution of the handover to the target basestation, the mobile terminal executes the handover to the target basestation including performing a contention-based RACH procedure based ona random access preamble randomly selected by the mobile terminal.

Advantageously, this further implementation enables the (e.g. target)base station to reduce the resource allocation demands in the sense thatrandom access preambles (and associated RACH resources) are onlyuniquely assigned to mobile terminals for a limited amount of time,namely for the second pre-configured period of time. After elapse of thesecond pre-configured period of time the (e.g. target) base station isgiven the opportunity to reassign the same random access preamble to adifferent mobile terminal. In this respect, the time between subsequentassignments of contention-free random access preambles can be improved.

As can be readily appreciated, the further implementation should not beunderstood in the sense that it is limited to the first embodiment,namely to handover commands messages including handover executionconditions, only. There might be various other scenarios in which themobile terminal receives a handover command message (i.e. withouthandover execution condition) but is not immediately capable ofperforming the contention-free RACH procedure. In these scenarios, asecond pre-configured period of time would allow for the same advantagenoted above.

According to a modification of the above noted further implementation,the received handover command message includes an RACH validity timerbased on which the second pre-configured period of time may be set.Moreover, upon reception of a handover command message including theRACH validity timer by the mobile terminal, it is adapted tore-configure the second pre-configured period of time based on thereceived RACH validity timer.

According to an even further implementation of the first embodiment,further mechanisms shall be defined in response to which the mobileterminal discards the received handover command message.

Firstly, the handover command message may, as part of the handoverexecution condition, include a further threshold value for the signalstrength or signal quality of the source radio cell, where, upon themobile terminal determining that the respective signal strength orsignal quality of the source radio cell rises above this furtherconfigured threshold values, the mobile terminal immediately discardsthe received handover command message (i.e. it does not wait for expiryof the first pre-configured period of time or the handover validitytimer).

Secondly, in case the mobile terminal is (e.g. still) performing thedetermination step based on a receive handover execution conditionincluded in an old handover command message, and at the same timereceives a new (e.g. fresh) handover command message, the mobileterminal also immediately discards the old handover command message andstarts the method for performing the handover based on the new receivedhandover command message.

Second Embodiment

Referring now to the second embodiment of the invention, variousimplementations of improved configurations of measurement cycles are tobe discussed in connection with FIGS. 15 to 16. Specifically, FIG. 15illustrates a sequence diagram of the configuration of measurementcycles of a mobile terminal by a master base station.

In the context of the second embodiment it is assumed that the mobileterminal, for which the measurement cycles are configured by the masterbase station, is connected via a macro radio cell to the master basestation and via a small radio cell to the secondary base station.

For configuration of the measurement cycles, the master base station,according to the second embodiment, determines (c.f. Step S1501 in FIG.15) a geographical distance between the master base station and thesecondary base station both serving the mobile terminal. As alreadypointed out at the beginning of the detailed description, the term pathloss may be used as an equivalent to the term geographical distance.

For instance in FIG. 16, an exemplary deployment scenario of a masterbase station (macro cell) and two secondary base stations (small cell 1and 2) is illustrated, where one secondary base station (small cell 1)is located at a small geographical distance from the master base station(i.e. close to the center of the macro radio cell). Another secondarybase station (small cell 2) is located at a large geographical distancefrom the master base station (i.e. at the perimeter of macro radiocell).

Then, the master base station compares (c.f. step S1502 in FIG. 15) thedetermined geographical distance to a pre-configured distance threshold.The distance threshold is pre-configured such that a mobile terminal,being connected to the master and the secondary base station,experiences the same or similar signal properties with respect to themaster and the secondary base station. In other words, based on thepre-configured threshold it can be ensured that in case a mobileterminal is connected via the small radio cell to the secondary basestation it will not loose coverage by the macro radio cell.

Subsequently, the master base station generates (c.f. step S1503 a orS1503 b in FIG. 15) a measurement control message for the mobileterminal including a first measurement object indicating a measurementcycle for the macro radio cell and a second measurement objectindicating a measurement cycle for the small radio cell. Thismeasurement control message is thereafter transmitted (c.f. step S1504in FIG. 15) by the master base station to the mobile terminal forconfiguring measurement cycles of the mobile terminal.

In particular, in the second embodiment, the measurement cycle withinthe transmitted first measurement object for the macro radio cell is setby the master base station based on a result of the comparison of thedetermined geographical distance to the pre-configured distancethreshold. Accordingly, in case the master base station determines themaster and the secondary base stations to be at close geographicaldistance, i.e. below the pre-configured distance threshold, the masterbase station allows (c.f. step S1503 a in FIG. 15) the mobile terminalto artificially prolong the measurement cycle for the macro radio cell(i.e. measurements are carried out at exceptionally long intervals).

Similarly, in case the master base station does not determine the masterand the secondary base stations to be at close geographical distance,i.e. below the pre-configured distance threshold, the master basestation also generates a measurement control message (c.f. step S1503 bin FIG. 15) however, it does not allow the mobile terminal toartificially prolong the measurement cycle for the macro radio cell(i.e. measurements are carried out at exceptionally long intervals).

Due to artificially lengthy measurement cycles for the macro radio cell,the power consumption in the mobile terminal can be reduced. Inparticular, in case of dual connectivity where the mobile terminal isoffloaded to the secondary radio cell (i.e. small cell offloadingsituation), the mobile terminal does not have to continue withmeasurements for the macro radio cell at regular intervals but canconfigure a prolonged measurement cycle for the macro radio cell.

In other words, conventionally the measurement cycle is set for anyradio cell so that a mobile terminal does not lose coverage thereby. Inthis respect a prolonged measurement cycle would result in the risk oflosing coverage by the respective radio cell. In contrast thereto, theinformation that the master and secondary base station have ageographical distance being smaller than the pre-configured distancethreshold is utilized by the master base station for enabling aprolonged measurement cycle for the macro radio cell without risk oflosing coverage thereby.

Referring back to the exemplary deployment scenario illustrated in FIG.16, when a mobile terminal is located at close proximity to thesecondary base station (small cell 1), the master base stationdetermines that same secondary and the master base station are locatedat a geographical distance which is smaller than the pre-configuredthreshold value. In this situation, the master base station generatesand transmits to the mobile terminal a measurement control messageindicating a regular measurement cycle for the small radio cell and aprolonged measurement cycle for the macro radio cell.

Further, when the mobile terminal moves to a location where it is atclose proximity to the other secondary bases station (small cell 2), themaster base station determines that the other secondary and the masterbase station are not located at a geographical distance which is smallerthat the pre-configured threshold value. In this respect, the masterbase station does not generate and transmit to the mobile terminal ameasurement control message indicting a prolonged measurement cycle forthe macro radio cell.

First Implementation

According to a first implementation of the second embodiment, in casethe result of the comparison indicates that the determined geographicaldistance is smaller than the pre-configured distance threshold, themaster base station sets the measurement cycle for the macro radio cellto a value which indicates measurement relaxation to the mobileterminal.

Alternatively, the master base station sets the measurement cycle forthe macro radio cell to a value which corresponds to more than 5measurement cycles of the small radio cell. As another alternative, themobile terminal sets the measurement cycle for the macro radio cell to avalue which indicates deactivation of the measurements.

In all three cases, the mobile terminal does not have to continue withmeasurements for the macro radio cell at regular intervals but canconfigure a prolonged measurement cycle for the macro radio cell,thereby reducing the power consumption by the mobile terminal.

Second Implementation

With respect to the determination of the geographical distance betweenthe master and the secondary base station, according to a secondimplementation of the second embodiment, the master base station looksup the geographical position of the secondary base station from alocation information table stored in the master base station.Alternatively, the master base station receives from the secondary basestation location information indicating the geographical positionthereof. As another alternative, the master base station measures thesignal strength of reference signals that are transmitted by thesecondary base station via the small radio cell.

In these three alternatives, the master base station is provided withinformation on the geographical distance between the master and thesecondary base station and hence may use this information in thesubsequent configuration of measurement cycles of the mobile terminal.

Alternatively, the mobile station may be configured to determine thegeographical distance based on received measurement reports from the oneor from plural mobile terminal(s) in dual connectivity with the masterand the secondary base station. The measurement reports indicate signalstrengths of reference signals that are transmitted by the secondarybase station via the small radio cell and indicating signal strengths ofreference signals that are transmitted by the master base station viathe macro radio cell. In this respect, the master base station maydetermine whether or not the secondary base station is located undercoverage by the macro radio cell (e.g. reference reports indicatesimilar signal strengths). Alternatively, the master base station maydetermine the geographical distance by the master base station measuringa round-trip-time for information transmitted to and received from thesecondary base station. In these later two alternatives an approximatedgeographical distance between the master and the secondary base stationis determined.

In any case variations of the first implementation and secondimplementation may be combined with each other to arrive at aconfiguration of measurement cycles of a mobile terminal which providesfor the advantages named above.

Hardware and Software Implementation of the Invention

Another embodiment of the invention relates to the implementation of theabove described various embodiments using hardware and software. In thisconnection the invention provides an user equipment (mobile terminal)and an eNodeB (base station). The user equipment is adapted to performthe methods described herein.

It is further recognized that the various embodiments of the inventionmay be implemented or performed using computing devices (processors). Acomputing device or processor may, for example, be general purposeprocessors, digital signal processors (DSP), application specificintegrated circuits (ASIC), field programmable gate arrays (FPGA) orother programmable logic devices, etc. The various embodiments of theinvention may also be performed or embodied by a combination of thesedevices.

Further, the various embodiments of the invention may also beimplemented by means of software modules, which are executed by aprocessor or directly in hardware. Also a combination of softwaremodules and a hardware implementation may be possible. The softwaremodules may be stored on any kind of computer readable storage media,for example RAM, EPROM, EEPROM, flash memory, registers, hard disks,CD-ROM, DVD, etc.

It should be further noted that the individual features of the differentembodiments of the invention may individually or in arbitrarycombination be subject matter to another invention.

It would be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects to be illustrative andnot restrictive.

The invention claimed is:
 1. A communication apparatus for performing ahandoff to a target base station, wherein the communication apparatus,under control of the target base station, communicates with the targetbase station via a target radio cell comprising a downlink carrier andan uplink carrier, the communication apparatus comprising: a receivingcircuit configured to receive a handoff command message, for the handoffto the target base station, including a handoff execution condition; anda processor configured to determine, based on the received handoffexecution condition included in the handoff command message, whether ornot the communication apparatus is to trigger execution of the handoffto the target base station; wherein, in case the processor determinesthat the communication apparatus is to trigger execution of the handoffto the target base station, the communication apparatus executes thehandoff to the target base station (a) by performing a random accesschannel (RACH) procedure with the target base station to thereby timealign the uplink carrier of the target radio cell, and (b) bytransmitting to the target base station a handoff complete messagethereby indicating completion of the handoff to the target base stationincluding completion of configuration of the communication apparatus forcommunication with the target base station via the target radio cell,wherein the handoff execution condition, included in the receivedhandoff command message, corresponds to an indication to triggerexecution of the handoff based on threshold values for signal strengthor signal quality of a source and/or target radio cell; and theprocessor, in operation, determines whether or not the signal strengthor signal quality of the source radio cell falls below one of thethreshold values and/or the signal strength or signal quality of thetarget radio cell rises above another of the threshold values; andwherein, upon the signal strength or signal quality having fallen belowthe one of the threshold values and/or having risen above the another ofthe threshold values, the communication apparatus is to triggerexecution of the handoff to the target base station.
 2. Thecommunication apparatus according to claim 1, wherein the communicationapparatus, prior to receiving the handoff command message, communicateswith a source base station via the source radio cell; and the receivingcircuit, in operation, receives the handoff command message from thesource base station via a downlink carrier of the source radio cell;and, after receiving the handoff command message, the communicationapparatus detaches from the source radio cell whereby communication withthe source base station is stopped.
 3. The communication apparatusaccording to claim 1, wherein the handoff execution condition, includedin the received handoff command message, corresponds to a timer value,and the processor, in operation, determines whether or not a timer withthe timer value has expired; wherein, upon expiration of the timer withthe timer value, the communication apparatus is to trigger execution ofthe handoff to the target base station.
 4. The communication apparatusaccording to claim 1, wherein the processor, in operation, determineswhether or not the signal strength or signal quality of the source radiocell falls below one of the threshold values and/or the signal strengthor signal quality of the target radio cell rises above another of thethreshold values, and wherein, upon the signal strength or signalquality having fallen below the one of the threshold values and/orhaving risen above the another of the threshold values, thecommunication apparatus is to trigger execution of the handoff to thetarget base station.
 5. The communication apparatus according to claim1, wherein the handoff execution condition, included in the receivedhandoff command message, corresponds to at least one threshold value forthe signal strength or signal quality of the source and/or target radiocell, and the processor, in operation, determines whether or not thesignal strength or signal quality of the source radio cell falls belowthe at least one threshold value and/or the signal strength or signalquality of the target radio cell rises above the at least one thresholdvalue, and wherein, upon the signal strength or signal quality havingfallen below the at least one threshold value and/or having risen abovethe at least one threshold value, the communication apparatus is totrigger execution of the handoff to the target base station.
 6. Thecommunication apparatus according to claim 5, wherein the thresholdvalue for the signal strength corresponds to a threshold value for areference signal received power (RSRP) to be measured by thecommunication apparatus based on reference signals (RSs) transmitted inthe source and/or target radio cell, and the threshold value for thesignal quality corresponds to a threshold value for a reference signalreceived quality (RSRQ) to be measured by the communication apparatusbased on reference signals (RSs) transmitted in the source and/or targetradio cell.
 7. The communication apparatus according to claim 1, whereinthe handoff execution condition, included in the received handoffcommand message, corresponds to an indication to trigger execution ofthe handoff based on out-of-sync events where a channel quality of thesource radio cell falls below an out-of-sync threshold (Qout), and theprocessor, in operation, determines whether or not a number ofout-of-sync events exceeds a pre-configured number N, and wherein, incase the number of out-of-sync events exceeds the pre-configured numberN, the communication apparatus is to trigger execution of the handoff tothe target base station.
 8. The communication apparatus according toclaim 1, wherein the handoff execution condition, included in thereceived handoff command message, corresponds to a counter value forout-of-sync events where a channel quality of the source radio cellfalls below an out-of-sync threshold (Qout), and the processor, inoperation, determines whether or not a number of out-of-sync eventsexceeds a received counter value, and wherein, in case the number ofout-of-sync events exceeds the received counter value, the communicationapparatus is to trigger execution of the handoff to the target basestation.
 9. The communication apparatus according to claim 1, whereinthe handoff command message corresponds to aRRCConnectionReconfiguration message.
 10. The communication apparatusaccording to claim 1, wherein the handoff complete message correspondsto a RRCConnectionReconfigurationComplete message.
 11. The communicationapparatus according to claim 1, wherein: in case the processordetermines, within a second pre-configured period of time, that thecommunication apparatus is to trigger execution of the handoff to thetarget base station, the communication apparatus executes the handoff tothe target base station including (a) performing a contention-free RACHprocedure based on a random access preamble included in the handoffcommand message; and in case the processor determines, after expiry ofthe second pre-configured period of time, that the communicationapparatus is to trigger execution of the handoff to the target basestation, the communication apparatus executes the handoff to the targetbase station including (a) performing a contention-based RACH procedurebased on a random access preamble randomly selected by the communicationapparatus.
 12. The communication apparatus according to claim 1, whereinthe received handoff command message includes a RACH validity timer, andthe processor, in operation, re-configures the second pre-configuredperiod of time based on the RACH validity timer.
 13. A method forperforming a handoff of a communication apparatus to a target basestation, wherein the communication apparatus, under control of thetarget base station, communicates with the target base station via atarget radio cell comprising a downlink carrier and an uplink carrier,the method comprising: receiving by the communication apparatus ahandoff command message for the handoff to the target base stationincluding a handoff execution condition; and determining by thecommunication apparatus, based on the received handoff executioncondition included in the handoff command message, whether or not thecommunication apparatus is to trigger execution of the handoff to thetarget base station, wherein, in case the communication apparatusdetermines that it is to trigger execution of the handoff to the targetbase station, the communication apparatus executes the handoff to thetarget base station (a) by performing a random access channel (RACH)procedure with the target base station to thereby time align the uplinkcarrier of the target radio cell, and (b) by transmitting to the targetbase station a handoff complete message thereby indicating completion ofthe handoff to the target base station including completion ofconfiguration of the communication apparatus for communication with thetarget base station via the target radio cell, and wherein the handoffexecution condition, included in the received handoff command message,corresponds to an indication to trigger execution of the handoff basedon threshold values for signal strength or signal quality of a sourceand/or target radio cell; and determining whether or not the signalstrength or signal quality of the source radio cell falls below one ofthe threshold values and/or the signal strength or signal quality of thetarget radio cell rises above another of the threshold values, wherein,upon the signal strength or signal quality having fallen below the oneof the threshold values and/or having risen above the another of thethreshold values, the communication apparatus is to trigger execution ofthe handoff to the target base station.